This invention relates to apparatus and method for accurately and uniformly cooling a hot, extruded polymeric sheet, and for providing auxiliary cooling for the sheet under mechanically stable, self-supporting conditions and without sagging or distortion.
Polymeric extruded or coextruded sheets are usually extruded out of a slit die of appropriate width, as indicated in the U.S. Patents to Frank R. Nissel U.S. Pat. Nos. 3,883,704, 3,918,865, 3,940,221, 3,959,431 and 4,533,510, for example. The hot sheet is then cooled by passing it through a pair or a series of temperature-controlled rolls. Although various roll numbers and arrangements may be used, three rolls are often used, sometimes in vertical planar alignment with each other. The gaps between the rolls are desired to be precisely adjusted according to the desired final sheet thickness. This precision is necessary for a variety of reasons, including elimination of air entrapment between the rolls, which causes adverse or uneven heat transfer or cooling.
Extruded plastic sheet is usually cooled with a vertically arranged three-roll stack wherein the middle roll is fixed in position but rotatable and the two upper and lower rolls are movably arranged to press gently against the sheet passing around the middle roll. This pressure assures air free contact with the rolls contacting the sheet for improved cooling.
In the field of plastic sheets, extrusion dies having variable lip portions have been used to form polymeric sheets of varying widths and thickness. Once extruded through an adjustable lip die, the polymeric sheet is substantially at its desired thickness. However, there is a demand for rapid cooling of a preformed polymeric sheet without excessively stressing the polymeric sheet product.
With increasing extrusion capacity, these cooling rolls have to be increased in diameter to provide sufficient cooling of the sheet. This diameter cannot always be increased sufficiently to assure adequate cooling of the sheet before leaving the last roll under mechanically stable, self-supporting conditions because of the increase of the die discharge distance from the first roll entrance. This excessive distance sometimes can cause sheet sagging and distortion prior to cooling, among other problems, because of the fluidity of the molten polymer. Fluid polymers like polyester are especially difficult to cool; they come out of the die with a consistency like liquid tar and are not very self supporting.
It has now been discovered that there is great advantage in adding a further roll (or set of rolls) to the system to assure sufficient cooling of the sheet. Such a cooling roll (or rolls) follow the third roll of the roll stack, but are not pressed against it because the gap would interact with the position of the third roll with respect to the second. It is, however, desirable to maintain a minimum gap between the bottom roll and each such auxiliary roll to minimize distortion of the still soft sheet between rolls. On the other hand, a substantial gap is required to thread the sheet through the roll train safely during start-up.
It is an object of this invention to create an improved apparatus and method of achieving a wide roll gap for threading with a closely controlled roll gap for accurate processing while using one or more auxiliary cooling rolls.